In prior art electrostatic coating apparatuses, particularly those for painting new motor vehicle bodies, a high voltage potential is applied to the spraying head of a rotary atomizer to produce an electrical charge in the sprayed coating material for improved adherence to a grounded workpiece. A problem has arisen, however, when a coating material of relatively good conductivity is used, for example a so-called water enamel. This problem results from the electrical insulation resistance in the conduit line connecting the spraying head with the paint supply system being too low if the paint supply system is at electrical ground potential. That is, the electrically conductive coating material inside the conduit lines will provide an electrical path between the high voltage spraying head and the grounded supply system.
In order to overcome this problem, it is theoretically possible to electrically insulate the entire supply system from electrical ground. This solution is practically undesirable, however, if the supply system includes of a plurality of storage vessels, each storage vessel supplying a different color of paint. Apart from the considerable cost of insulating the entire storage system, there is the possibility that an extensive supply system may have such a large capacity that there is a danger of explosive discharges at the spraying head. Furthermore, if the storage vessels are electrically insulated from ground, and thus maintained at the same high electrical potential as the spraying head, the tank can not be topped up, i.e., refilled, with paint without switching off the electrical current, unless costly additions such as intermediate tanks or the like are provided. Reference of such may be had in German Patent No. 29 00 660. In addition to this, many known systems require costly high output and high voltage sources.
According to another known method of overcoming the problem in question, the entire paint supply system, from the storage tank to the atomizer spraying head, is electrically grounded while the sprayed coating material is indirectly charged by external electrodes surrounding the spraying head, as shown in the prior art foreign patent EP No. OS 0171042. This method, however, may not be implemented on coating apparatuses where the coating material is directly charged through the spraying head.
In the prior art German Patent No. OS 30 14 221, each paint color to be electrostatically sprayed is provided with its own storage vessel. Each vessel is insulated from electrical ground and from the other vessels, and supplies the high potential spraying device through a paint color changer and connecting conduit line. At the end of the coating process with a given color, and before changing to another color, the connecting line is flushed with a solvent, e.g., water, and is dried with compressed air to maintain the required electrical insulation between the ground potential color changer and the high potential spraying device. However, this installation is costly and bulky, especially when a large number of different storage vessels are involved.